The matter of the correct order of applying sharpening has been discussed various times (I will pick this question as an example, but it's not the only one), with a clear consensus which can be summed like: "apply it as the last step" which makes sense, since you are trying to make up for details which are not "really" present in the raw image (due to AA filter, for instance, or to inherent features of the camera sensors).
Furthermore, Lightroom is defined as a linear editor: if you want to "non linearly" remove a previous step you have to resort to a layer-supporting software, like the Gimp, Photoshop, or what else you favor.

But overall, you don’t have to worry about the timing of sharpening in the context of a Lightroom workflow, in large part because all of the “real” optimization work on your images doesn’t actually apply until you export the photo in some way, and Lightroom is intelligent about that process. Sharpening can be applied in the Develop module at any time, and then use the output sharpening options when preparing an image for final output.

What exactly is this "intelligence"? I imagine that if I were to over sharpen in the first stages, then reduce noise and sharpen again (duly reducing the image quality, but assume for the sake of it that I'm aiming for that) LR should not be able to "condense" the sharpening steps and apply them as a final step, since that would "void" the (admittedly silly) point of my edits.

I tried to interpret this as something like "if LR is able to prove that you are not playing silly tricks with sharpening (for instance you only have a single batch of sharpening edits and a single batch of noise reduction edits?) it will intelligently reorder them and apply as a final step.

Then I thought about the (not well named IMHO) import setting called "None", which is the default and the one that I had been using for a while before picking "Zeroed" which, as far as I can't tell, really doesn't apply anything. This "None" setting applies by default a moderate amount of sharpening (in LR 4 it is amount 25, radius 1.0, detail 25, masking 0), together with color noise correction (25).

So it seems that in its default setting LR is applying sharpening as a FIRST step, which would not be a smart move on the part of the smart LR engineers if it was such a killer for Image quality.

So my question is fourfold:

(once again) is the order of sharpening IN LIGHTROOM (as of version 4 if it matters) really important and in which situations?

is the use of the Zeroed setting a good idea to avoid a "first step sharpening"?

is the choice of the "None" setting with its default sharpening an issue, a non-issue, or what?

is LR really a linear editor or it has the habit of reshuffling (without notice) the order of the edits and I misunderstood the meaning of "linear"?

The kind of sharpening you want to avoid until the last step is output sharpening. In a non-destructive workflow where you can modify your input sharpening options at any point in time, I do think the order is less important. I would expect the most complicating factor to be adjustment brushes, as they probably are applied after other adjustments.
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HenrikDec 26 '12 at 13:44

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What a coincidence. I have just tried to make a part of image fully black (I've brushed it and set exposure to minimum) and then clone some other part of image onto that black area. Sadly, it was still black. So LR first applied Clone and then Brush, even that I did it in reverse order.
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Petr ÚjezdskýDec 26 '12 at 21:20

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@Petr: you made a player to darken that area that persist until you choose to "flatten" the image after which you can work on it. So it is not so much about doing things in "order", but you clone it and you still have the active layer that darkens it.
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Michael NielsenJan 9 '13 at 6:42

3 Answers
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Lightroom (or any other non-distructive editing program) does not edit your image.

It keeps your actions in a "separated area" (usually a sidecar file in XML format cached in memory) sometimes called "recipe" which keeps your last settings. For example I have a photo and I do the following edits in this order:

Brightness: +10

Contrast: +7

Sharpness: +3

Brightness: -2

Contrast: +5

Sharpness: -2

After each step LR will update the recipe and each recipe change will trigger the screen repainting of the same actual (raw) data and not a change of the photo itself.

Let's see how the algorithm works step by step, based on the edits above: (in the list below B means Brightness, C - Contrast, S - Sharpness).

That's why if you open the actual (raw) file with another program which doesn't have access / doesn't know how to interpret the recipe (even if there is an XML standard for that) the image will appear untouched.

When you "export" the file, in fact is applied the same pipeline as above IOW is taken the current recipe, applied to the original raw file and the resulting offscreen bitmap instead to be rendered (painted) on the screen it is saved in a file.

As you see, because these programs doesn't work with the actual photo (like e.g. Photoshop, PhotoPaint, GIMP etc.) the order in which modifications are applied is lost, the program editing engine being responsible of applying them in the what it thinks to be "correct" order.

So Tim Grey was right. Overall, it doesn't matter when you do your sharpening. Also, to clarify, "Zeroed" and "None" are just some recipes applied by default to all imported images from a batch to give you a starting point. You can freely change them. It doesn't lose anything.

Thanks @john thomas. The way in which non destructive editing works was clear to me. My question is about your next-to-last paragraph: "the program's editing engine being responsible of applying them in the what it thinks to be correct order". So really in LR you can sharpen when you want and how many times you want without fear of degradation, hoping that the engine is able to see-through the potentially interrelated steps of sharpening (with its halos) and noise reduction, image rotation (which must move each pixel, including those added by sharpening...) and so on.
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FrancescoJan 10 '13 at 9:55

but does it have a function to "freeze" the current content and then apply the following edits on this intermediate step? Say, it will not increase contrast on sharpness artifacts, but this is actually what you want for creative or educational purposes - can you then incorporate the sharpening into to pixels and reset the recipe to zero lines without doing an "export" and reopen that?
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Michael NielsenJan 10 '13 at 12:00

@Francesco: The engine will always 'see-through' - it will apply only the last settings. The settings are cumulated outside of the picture data, not inside. So, you really cannot do sharpening (or noise reduction) multiple times hence the artifacts will not add up.
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John ThomasJan 10 '13 at 16:44

@MichaelNielsen: No it hasn't, in the way in which you mean. It has the snapshot (like in many other programs), but the snapshot is a saved recipe only. The artifacts are not added. For conformity, to create a new snapshot go to menu 'Develop' and choose 'New Snapshot' or simply press Ctrl+N. Also you have Virtual Copies which are very neat but aren't what you are asking.
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John ThomasJan 10 '13 at 16:57

John, I think that @MichaelNielsen has precisely got the point of my question (which I don't think will receive much further clarification unless someone from LR team steps in...): what happens if I increase the contrast after having sharpened (producing artifacts)? nothing, if the engine decides to apply them later: a lot of image deterioration if the engine doesn't re-order them. Which could explain the behaviour observed by Petr in his comment. As soon as possible I will do some experiment on some test image to convince myself....lately I am really short on free time :-(
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FrancescoJan 10 '13 at 17:47

To answer your basic question, yes I believe Lightroom applies its settings in a pre-determined order, as determined by the Lightroom Engineers. I don't know if the exact order of effects has ever been published, but you can probably guess roughly.

Again, just a guess, but I think that the primary sharpening (in the sharpening panel) is done early in the process, but after noise removal. There is a secondary output sharpening during export, which I'm pretty sure is done just about last.

The overall processing of Lightroom is considered among the best out there (with various raw processors having strengths and weaknesses) so I think the Adobe engineers are doing something right.